A ring laser employs two beams of light propagated along the same path in opposite directions around a closed path reentrant cavity. In an ideal ring laser, the frequency difference between the beams of light is zero when the ring is stationary but moves from zero when the ring is rotated about an axis perpendicular to the lasing plane, the frequency difference being proportional to the angular rotation rate of the cavity. The two beams are combined using a prism to produce an interference pattern of "fringes". The fringes are stationary if the two beams are of identical frequency and have a constant phase displacement. The fringes change at a rate proportional to any difference in frequency of the two beams A photodiode detects the motion and provides a corresponding electrical signal. As such, a ring laser is capable of functioning as a rate gyroscope. However, there are many effects that degrade ring laser performance. One of the most dominant, and hence troublesome, effect is known as lock-in. Lock-in is caused by light scattered from a beam interacting with the oppositely propagating beam. One effect of this interaction is the suppression, at low rotation rates, of the frequency difference between the beams. Another effect of this interaction causes the frequency difference vs input angular rate to exhibit non-linear behavior and increased phase noise for input rates near the lock-in threshold.
When a ring laser is used as a gyroscope, known as an RLG, the two output light beams are combined to provide interference fringes which are counted by a photodetector. The fringe count is directly proportional to the total angle the ring laser has turned through, provided the two beams of light are completely uncoupled. The ratio of the fringe count per unit angle of rotation is known as the gyro scale factor. However, as a result of lock-in, no fringes will occur up to the lock-in threshold and the scale factor will be non-linear for a range of rotational rates near the lock-in threshold. As can be realized both of these phenomena seriously degrade the accuracy of the RLG.
Various techniques to avoid lock-in have been employed, ranging from a mechanical dithering arrangement that oscillates the entire ring laser at a small amplitude to magneto-optical biasing arrangements. The magneto-optical method imparts a bias to the ring laser by introducing a non-reciprocal phase shift to the counter-propagating light beams. Magneto-optical arrangements generally fall into two broad categories, namely Faraday cells and magnetic bias mirrors. In the Faraday cell biasing devices, a paramagnetic or ferrimagnetic material, transparent to the laser wavelength, is inserted in the cavity in the paths of the two light beams. This arrangement suffers the disadvantage that additional high quality, and hence expensive, optical components need be employed and furthermore, these components may give rise to increased light scatter thereby increasing the lock-in problem.
Patents related to this technology include U.S. Pat. 3,649,931, Mar. 14, 1972 to Macek entitled "Compensated Frequency Biasing System for Ring Laser" which shows the use of a Faraday bias cell system. U.S. Pat. No. 3,862,803, Jan. 28, 1975, to Yntema et al. entitled "Differential Laser Gyro System" also shows the use of a Faraday cell. A U.S. Pat. No. 3,973,851, Aug. 10, 1976 to Ferrar entitled "Dispersion Compensated Laser Gyro (U)" provides an axial magnetic field to a laser gain medium and, through the Zeeman Effect, provides a displacement between gain versus frequency profiles for counter-rotating waves.
A magnetic bias mirror typically replaces one of the usual three "corner" mirrors of the RLG in a manner disclosed in, for example, commonly assigned U.S. Patent Application Ser. No. 07/239,724, filed Sep. 2, 1988, entitled "Ring Laser Gyro and Magnetic Mirror Therefor", H. Lim et al. In use, a rapidly switchable magnetic field is generated by conductors near and preferably embedded in a mirror substrate. The S-mode polarization is suppressed by the use of a small perimeter, odd-number of mirrors configuration and/or with multilayer dielectric coatings. The resulting single P-mode operation is achieved without Brewster angle windows and the attendant birefringence and increased scatter problems.
This successful technique for avoiding lock-up phenomena in an RLG employs magnetic "dithering" obtained from a thin magnetic coating that forms a reflective surface of the mirror. This technique involves operating the RLG to produce only a P-polarized beam of light so that a transverse Kerr effect interaction of the light and the magnetic field at the magnetic thin film coated surface results in a non-reciprocal phase shift of the light travelling in the two counter-propagating beams. A consequence of this phase shift is a frequency split between the two beams when the gyro is unmoving in its inertial frame. Without the "magnetically induced bias" imparted by the magnetic mirror (MM) the RLG would produce zero output frequency until an associated lasing plane is rotated in inertial space at an output rate greater than the lock-in threshold. The magnetic bias provided by the MM overcomes this problem by maintaining the RLG out of the lock-in state.
As such, and as can be seen in FIG. 2, the MM effectively offsets the curve of output frequency vs. gyro rotation rate by an amount equal to the magnetically induced bias. When the MM is fully set in one of its two possible stable directions along an associated "easy" axis (determined during magnetic film deposition), the bias induced by the MM shifts the curve to a more positive angular rotation rate. When the MM is fully set to the opposite stable state along the easy axis, the curve is shifted to a more negative angular rotation rate. The lock-in region is correspondingly offset to straddle the bias value. The behavior for the two states of MM is symmetrical in regard to the magnetically induced bias produced. One advantage of this symmetry is a short term cancellation of a slowly varying bias magnitude due to, for example, temperature variations in the magnetic film, the short term cancellation being obtained through the use of a procedure that calculates the actual or true rotational rate by summing the total fringe counts over each full cycle of the mirror switching frequency.
However, it has been observed that for rotation rates in the vicinity of the MM bias offset, where lock-in tends to occur, two undesirable effects result: (1) the random noise output from the gyro tends to increase greatly, and (2) the actual output frequency deviates significantly from a desired straight line relationship of frequency versus input rotation rate.
One solution to this problem in order to maintain a low noise level involves stopping the symmetrical switching of the MM when the rotation rate nears one of the two MM bias offset values, and leaving the MM switched to a state which is farthest from lock-in.
However, in operation the symmetry of bias offset and cancellation of bias variations due to temperature and other variables is not available to the system employing the RLG while the MM is not being switched. To eliminate errors, any such variations are therefore required to be "modelled" and included in a correction algorithm run by a true rate, or navigational, computer. Such compensation or correctional techniques add to the complexity and could reduce the accuracy of the rotational rate sensing system.
Other patents of interest include the following. A U.S Pat. No. 4,592,656, Jun. 3, 1986, to Egli discloses a ring laser angular rate sensor having laterally positionable mirrors 2 and 3 for modulating scattered waves at a constant rate in integer multiples of 2(pi) radian phase change. In U.S. Pat. No. 4,410,276, Oct. 18, 1983, Ljung et al. discloses a ring laser gyroscope constructed as an isosceles triangle with two symmetrical mirrors arranged to reflect light at an angle such that equal and opposite vibration of the two mirrors relative to an area within the triangle can be carried out at an amplitude that corresponds exactly to one Bessel function zero. A third angle supplementary to but different from the first two angles is selected such that the displacement of the point of reflection on an associated reflector surface corresponds to another Bessel function zero.
The following two U.S. Patents generally describe magnetic mirrors. A U.S. Pat. No. 3,851,973, Dec. 3, 1974 to Macek entitled "Ring Laser Magnetic Bias Mirror Compensated for Non-Reciprocal Loss" and a U.S. Pat. No. 4,195,908, Apr. 1, 1980 to Kestigian et al. entitled "Magnetic Mirror for Imparting Non-Reciprocal Phase Shift".
A U.S. Pat. No. 4,522,496, Jun. 11, 1985 to Sanders entitled "Laser Gyro Mode Locking Reduction Scheme" describes in one embodiment the extraction of one of the primary waves from the ring laser path through a partially reflective mirror, the extracted wave being acted upon by a mirror oscillating at an angular frequency which doppler-shifts the frequency of the extracted wave. The doppler-shifted mode is reintroduced into the ring laser path to diminish the range of lock-in frequency of the primary modes.
The following patents generally describe various reflectors suitable for use in lasers or laser gyroscopes. A U.S. Pat. No. 4,442,414, Apr. 10, 1984, to Carter entitled "Magneto-Optical Phase-Modulating Devices", U.S. Pat. No. 4,009,933, Mar. 1, 1977, to Firester entitled "Polarization-Selective Laser Mirror", U.S. Pat. No. 4,268,799, May 19, 1981, to McCrickerd entitled "Curved Mirror Lasers and Methods of Operating Same", U.S. Pat. No. 4,201,954, May 6, 1980 to van der Wal et al. entitled "Gas Discharge Laser for Generating Linearly Polarized Radiation", U.S. Pat. No. 4,271,397, Jun. 2, 1981 to Stiles et al. entitled "Nonreciprocal Phase Shifter for a Ring Laser Gyro" and U.S. Pat. No. 3,581,227, May 25, 1971 to Podgorski entitled "Adjustable Thin Membrane Mirror for Use in the Stabilization of Ring Lasers". EP 267672A, entitled "Mirrors of Polarization Sensitive Reflectivity", describes a multi-layer dielectric stack mirror for a laser gyroscope.
However, none of the aforementioned patents either singularly or taken together either solves or suggests a solution to the problem of maintaining scale factor linearity or a low noise level when the angular rotation rate of a ring laser angular rotation rate sensor nears one of two MM bias offset regions.
It is thus an object of the invention to overcome the foregoing problems by enabling the RLG to operate at all desired rotation rates without closely approaching the lock-in region.
It is another object of the invention to overcome the foregoing problems by enabling a ring laser angular rate sensor to operate at all desired rotation rates without closely approaching the lock-in region by providing first and second magnetic mirrors within the optical cavity and by operating the two magnetic mirrors either in an additive mode or in a subtractive mode of operation.